1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor layer, a semiconductor device, a display device, a liquid crystal display device, or a light-emitting device. In addition, the present invention relates to a method for manufacturing a semiconductor device, a display device, a liquid crystal display device, or a light-emitting device. Alternatively, the present invention relates to a driving method of a semiconductor device, a display device, a liquid crystal display device, or a light-emitting device. In particular, the present invention relates to a semiconductor device, a display device, or a light-emitting device each including a transistor, or a driving method thereof, for example. Further, the present invention relates to, for example, an electronic device including the semiconductor device, the display device, or the light-emitting device.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all included in the category of the semiconductor device.
2. Description of the Related Art
A technique by which a transistor is formed using a semiconductor layer formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is known as a semiconductor film applicable to a transistor.
As the silicon film used as a semiconductor film of a transistor, either an amorphous silicon film or a polycrystalline silicon film is used depending on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferable to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferable to use a polycrystalline silicon film, which can form a transistor having a high field-effect mobility. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or laser light treatment which is performed on an amorphous silicon film has been known.
In recent years, an oxide semiconductor film has attracted attention. For example, a transistor which includes an amorphous oxide semiconductor film containing indium, gallium, and zinc is disclosed (see Patent Document 1).
An oxide semiconductor film can be formed by a sputtering method or the like, and thus can be used for a channel formation region of a transistor in a large display device. A transistor including an oxide semiconductor film has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. Moreover, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
A transistor including an oxide semiconductor film is known to have extremely low leakage current in an off state. For example, a CPU or the like with low-power consumption utilizing the low leakage current of the transistor including an oxide semiconductor film is disclosed (see Patent Document 2).
Patent Document 3 discloses that a transistor having high field-effect mobility can be obtained by a well potential formed using an active layer formed of oxide semiconductor layers.